Hybrid vehicles have recently been developed that include an internal combustion engine generating driving force from thermal energy produced by burning such a fuel as gasoline as well as an electric motor generating driving force from electrical energy. Even when the engine is stopped, the hybrid vehicle can run by driving the electric motor with electric power supplied from a battery that is charged in advance or charged with electric power generated by the engine while the vehicle is running.
When the hybrid vehicle runs by using the driving force from the engine, the electric motor is driven in an auxiliary manner to enhance combustion efficiency of the engine and thereby allow the engine to operate at a most efficient point all the time. Fuel consumption and the amount of carbon dioxide in exhaust gas can thus be reduced. Namely, low pollution and energy saving can simultaneously be achieved.
In the hybrid vehicle, the interior of the passenger compartment is heated by means of thermal energy derived from cooling water of the engine. Specifically, such a vehicle has a heating apparatus that supplies cooling water heated by waste heat from the engine to a heater core for heat exchange between the air to be blown into the passenger compartment and the cooling water.
As for the hybrid vehicle, however, heat generation from the engine is kept low since the engine is operated at the most efficient point, so that the temperature of the engine cooling water is also kept relatively low. Therefore, when high heating ability is required because of a low outside-air temperature, a problem arises that is insufficient heating ability of the configuration using the engine cooling water as a heat source.
In order to solve the aforementioned problem, Japanese Patent Laying-Open No. 09-233601 for example proposes a hybrid vehicle having necessary and sufficient heating performance. Specifically, when the hybrid vehicle is stopped or driven by the electric motor and the difference between a preset temperature of the air conditioner and a room temperature is equal to or larger than a criterion value that is determined according to the temperature of the engine cooling water, the internal combustion engine is forced to operate to obtain the above-described heating performance.
A similar technique is proposed for example by Japanese Patent Laying-Open No. 10-203145 that discloses a heating control apparatus for a hybrid vehicle. The hybrid vehicle has its engine operated at maximum efficiency. A temperature of the cooling water required for heating is used as a preset temperature. When the actual temperature of the cooling water does not reach the preset temperature, a request is made to change the operating point of the engine for the purpose of increasing the temperature of the engine cooling water.
The configuration of the hybrid vehicle disclosed in Japanese Patent Laying-Open No. 09-233601 ensures the heating performance by increasing the temperature of the cooling water through idling of the engine. Therefore, it takes some time to allow the temperature of the cooling water to reach a target value (preset temperature). Until the temperature of the cooling water reaches the preset temperature, the engine cannot be driven intermittently, resulting in increases in amount of exhaust gas and fuel consumption.
The configuration disclosed in Japanese Patent Laying-Open No. 10-203145 uses control for ensuring the heating performance by requesting a change of the operating point of the engine when it is necessary to increase the temperature of the cooing water. However, the control only makes a switch between the operation of changing the engine operating point and the operation of not changing the engine operating point. Thus, fine control based on the difference between the cooling water temperature and the preset temperature cannot be performed, and the time that is necessary for the engine water temperature to reach a target value cannot be made sufficiently shorter.